1. Field of the Invention
The present invention relates to an aqueous ink composition for use in an ink-jet printer and a pen plotter which are used to record information output by a computer, or transmitted by a facsimile apparatus, or which provide video images and photographic images; writing utensils such as ball point pens, marking pens and fountain pens; and various kinds of recorders employing a pen, a stamp or a pad.
2. Discussion of Background
An ink-jet recording method is capable of recording images by ejecting a liquid ink from a nozzle. This ink-jet printing method has the advantages that images can be recorded on a sheet of plain paper, color images can easily be produced and the printing speed is high, so that many kinds of ink-jet printers which work on various ink-ejection principles are commercially available.
For example, conventional ink-jet printers employ the following ink-ejection principles:
(1) By use of a piezoelectric element such as a PZT which is deformed by the application of a voltage thereto, pressure is applied to an ink contained in a tank to eject the ink from a nozzle.
(2) An electrostatic field controlled by turning an electrode on or off is applied to the ink, thereby ejecting the ink from a nozzle.
(3) Thermal energy is applied to the ink by use of a heating element, to change the phase of the ink resulting in the ink being ejected from a nozzle because of a change in the volume of the ink.
(4) A predetermined pressure is continuously applied to an ink by use of a pump, so that the ink is continuously ejected from a nozzle. During the continuous ejection of the ink, the charging conditions for each ink drop are controlled by an electrode disposed near the nozzle, thereby controlling the direction of ejection of the ink drops.
The ink for use in the above-mentioned ink-jet printer and writing utensils is required to contain a dye at a sufficiently high concentration to produce images with high image density. In addition, the requirements for the ink are as follows:
Produced images have high water-resistance.
The ink has high light-resistance.
The values of the physical properties of the ink, such as viscosity and surface tension, are within the proper ranges.
No precipitates are formed in the ink composition and the desired physical properties do not change during a long period of storage.
Normal recording can be carried out with a pen or ink-jet printer without clogging the pen point or printer nozzle with the ink even if the recording operation is resumed after a long intermission.
However, in general, dyes with good water-resistance lack solubility, so that an ink composition comprising a dye with high water-resistance clogs a nozzle. Furthermore, the higher the concentration of a dye in an ink composition, the more easily the nozzle is clogged. As previously mentioned, it is difficult to satisfy all the requirements for the ink composition at the same time.
To solve the above-mentioned problems, attention has been paid to the selection of a dye and a wetting agent to be contained in an ink composition and the addition of a dye-solubilizer and a surface active agent thereto. For instance, an ink composition comprises a dye which contains a sulfonic acid in the form of a lithium salt, as disclosed in Japanese Laid-Open Patent Application 57-202358; an ink composition comprises as a dye a quarternary-ammonium-ion-containing salt, as disclosed in Japanese Laid-Open Patent Application 62-149770; and an ink composition comprises as a dye a quaternary-phosphonium-ion-containing salt, as disclosed in Japanese Laid-Open Patent Application 63-48374.
Even though the aforementioned dyes have high water-resistance, and are contained in the respective ink compositions at a high concentration, the clogging of a pen point or a nozzle of the ink-jet printer can be avoided because the dyes are contained in the form of a salt in the ink compositions. However, most commercially available water-soluble anionic dyes are available in the form of a sodium salt. In order to exchange a cationic dye counter ion of the anionic dye in the form of a sodium salt with a desired cationic dye counter ion, it is necessary to carry out a complicated operation such as acidifying-out, treatment by use of an ion-exchange resin, or salting-out. The application of this kind of preparation procedure, however, is restricted from the viewpoint of coat.
The previously mentioned conventional ink-jet printing method employs an aqueous ink composition. The reason for this is that an aqueous ink composition is water-based and very safe for the user even if the user touches the ink composition, drinks it by mistake, or inhales the vapor from the ink composition. In addition, when the aqueous ink composition is used for recording images, sharp images can be obtained without being spread on a sheet of paper since the surface tension of water is higher than the surface tensions of other liquids for use in the ink composition.
However, the water-based aqueous ink composition lacks affinity for paper, especially for paper which has been sized, so that the penetrating speed of the ink through the paper is slow after the ink drops are deposited on the surface of the paper. Consequently, the apparent drying rate is slow. Therefore, recorded images are often impaired when touched or after coming into contact with transporting members and papers successively transported into a printer while the ink component stays wet on the paper. In addition, when a color image is produced on the paper by superimposing a plurality of colored inks, the color image is apt to be impaired at the boundaries of a secondary color obtained by superimposing the colored inks.
To solve the above-mentioned problems, it is proposed to use as a recording medium a sheet of paper which contains no sizing agent, or only a reduced amount of sizing agent, as disclosed in Japanese Laid-Open Patent Application 52-74340; and a recording paper comprising a surface layer which mainly comprises a white pigment and a water-soluble polymeric material, as disclosed in Japanese Laid-Open Patent Applications 52-5312 and 56-89594. The above-mentioned specific recording media can solve the above-mentioned problem. However, there is a demand for images recorded on ordinary plain papers for home or office use.
Furthermore, Japanese Laid-Open Patent Application 56-57862 discloses a method for increasing the drying characteristics of an ink. More specifically, the ink is adjusted to a pH of 12 or more by dissolving an alkaline compound therein at high concentration. Thus, a sizing agent for use in paper can be dissolved in the ink, so that the drying characteristics of the ink can be improved. According to this method, the penetrating speed of the ink through a so-called acidic paper, which has been sized by the use of rosin, can be increased to some extent. However, in the case where color images are recorded on the acidic paper by superimposing the colored inks, the impairing of recorded images at the boundaries of a secondary color obtained by superimposing the color inks cannot be prevented. This method is ineffective for a neutral paper which has recently been into common use. Another shortcoming of this method is that the life span of a head portion of an ink-jet printer used for ejecting the ink is shortened because the ink composition with a high pH value has adverse effects on a material used for the head of the ink-jet printer.
In order to improve the drying characteristics of such inks, an ink composition comprising an aliphatic monovalent alcohol is proposed, as disclosed in Japanese Laid-Open Patent Application 55-145774. In this case, the penetrating speed through the paper can be increased to some extent. However, when the aliphatic monovalent alcohol is contained in the ink composition at high concentration, a dye component separates out, clogging a nozzle of an ink-jet printer, and the problem of emitting an unpleasant odor often arises. On the other hand, when the concentration of the aliphatic monovalent alcohol is decreased, the drying speed of the resulting ink composition becomes insufficient to prevent the impairing of the recorded color images at the boundaries of the secondary color.
To increase the penetrating speed of the ink through the paper, the addition of a surface active agent to an ink composition has been proposed, as disclosed in Japanese Laid-Open Patent Application 55-29546. According to this method, the penetrating speed of the ink through the paper can be increased by using a proper surface active agent, so that the impairing of color images at the boundaries of the secondary color can be prevented. However, organic members such as plastics and adhesives for constructing a head portion of an ink-jet printer swell when used in contact with a commercially available surface active agent, or are dissolved in the surface active agent, resulting in the reduction of the life of the head portion. In addition, the clogging of a nozzle with an ink cannot always be solved by this method.
An aqueous ink composition for ink-jet printers, comprising a salt of perfluorosulfonic acid serving as an anionic surface active agent, is proposed in Japanese Laid-Open Patent Application 57-90070. According to this application, the potassium salt of perfluorosulfonic acid is preferable as a surface active agent. However, the objects of the present invention cannot be achieved by the aforementioned ink composition, which comprises the anionic surface active agent in the form of a salt comprising a potassium cation.